The conventional (single-junction) solar cells convert less than 20 % of sunlight into electricity. Tandem (multi-junction) solar cells, on the other hand, use more of the solar spectrum to improve efficiency. They utilize photon energy through multiple semiconductors and guide all photons into an absorber where the bandgap matches the photon energy.

Recently, the researchers at the UCLA Samueli School of Engineering designed a thin-sheet, double-layer solar cell that is far more efficient than signal junction solar cells. It uses photoactive materials with complementary absorption properties to leverage a wider solar spectrum.

It converts 22.4% of the sunlight into electrical energy – a new record in power conversion efficiency for this type of solar cell. The previous record was 10.9%, achieved by IBM in 2015.

How Did They Develop This?

To build this solar cell, researchers used a thin film of perovskite — a low-cost material that has the same crystal structure as the calcium titanium oxide mineral. Methylammonium lead iodide is a widely known synthetic perovskite with 31% theoretical maximum efficiency.

Since perovskites have a unique crystal structure, they show several intriguing properties such as massive magnetoresistance, ferroelectricity, and superconductivity. Moreover, their optical, electrical and physical characteristics can be easily designed and optimized.

Image credit: UCLA

The lower layer of the cell is made up of a compound of CIGS (short for copper, indium, gallium, and selenide). It’s nearly two microns thick and capable of generating electrical energy from sunlight with 18.7% efficiency on its own.

However, if you add one micron thick layer of perovskite, you can further increase the efficiency of solar cells. This is similar to improving car engine performance by integrating a turbocharger.

The researchers developed a nanoscale interface to combine these two layers. This interface provides a higher voltage, which further boosts the power device can export. The whole system sits on a two-millimeter thick glass substrate.

Since the new design draws energy from two different portions of the solar spectrum over the same device area, it enhances the power produced from solar energy compared to the single layer CIGS. The best thing is the process of adding a perovskite layer can be easily deployed to current solar-cell manufacturing methods.

The CIGS rear cells can be completely detached from perovskite front cells, and used separately. The rear cells maintain the same performance when the front cells are removed. This demonstrates that the fabrication as well as dissolving processes of perovskite do not damage the CIGS device.

Overall, the new technique increased the performance of the existing multi-junction solar cell by approximately 20%, meaning a 20% cut in energy costs. The researchers believe that it could ultimately achieve 30% power efficiency in coming years.